1. Field of the Invention
The present invention relates to a circuit for generating a reference voltage, more particularly to a bandgap reference voltage circuit.
2. Description of the Related Art
Bandgap reference voltage circuits are widely used because of their ability to generate a reference voltage that does not vary with temperature. FIG. 21 shows a bandgap reference voltage circuit described in, for example, Japanese Unexamined Patent Application Publication No. 11-231948. The circuit includes a reference stage 50 that generates a constant current proportional to a thermal voltage and generates the bandgap reference voltage from the constant current, a pair of start-up circuits 60A, 60B that start the reference stage 50 when power is initially applied, and a pair of filters 70A, 70B that filter the high power supply Vcc and lower power supply Vss.
During operation, p-channel transistors P500, P502, P508 form a first current mirror stage in the reference stage 50, p-channel transistors P504, P506, P509 form a second cascoded current mirror stage, and n-channel transistors N500, N502 also form a current mirror. All of these transistors operate in their saturation regions, due to the connections of their gate electrodes to nodes 517, 518, and 519. Resistor R500 enables the saturation state to be reached at a relatively low power-supply voltage. The current mirrors hold the currents on paths 512, 514, 516 to constant values determined by the sizes of bipolar transistors Q500 and Q502 and the value of resistor R502. The value of resistor R504 and the base-emitter voltage of bipolar transistor Q504 then establish a reference voltage Vref at node 510, which is held by capacitor C500 and made available to external circuits (not shown).
To generate the reference voltage Vref, it is necessary to initiate current flow on paths 512, 514, and 516, but the reference stage 50 is incapable of doing this by itself. The reason is basically that paths 512, 514, and 516 will not conduct until electrons have been supplied to or removed from the gates of transistors P500–P509, N500, and N502, but electrons cannot be supplied and removed via paths 512, 514, 516 until these paths conduct. This dilemma is overcome by having the first start-up circuit 60A draw electrons from the gates of transistors N500 and N502, and the second start-up circuit 60B supply electrons to the gates of transistors P500–P509. The start-up operation begins and ends as follows.
When the bandgap reference voltage circuit in FIG. 21 is initially powered up and the high power supply voltage Vcc rises, p-channel transistors P512 and P514 promptly turn on and supply Vcc to node 518, thereby turning on n-channel transistors N500 and N502. Since node 522 is initially at the low power supply voltage Vss, p-channel transistor P526 and n-channel transistor N508 turn on, supplying Vss to node 519 and turning on p-channel transistors P500, P502, and P508. Node 517 is also pulled down to the Vss level through resistor R500, turning on p-channel transistors P504, P506, and P509. Current can now flow on paths 512, 514, and 516, and a reference voltage Vref is generated.
When p-channel transistors P500–P509 turn on, p-channel transistors P516 and P518 in start-up circuit 60A also turn on, thereby supplying current to a disable node 520 and charging a connected capacitor C502. When the voltage at disable node 520 reaches such a level that the source-to-gate voltage of transistor P512 no longer exceeds the threshold voltage, transistor P512 turns off, ending the pulling up of node 518.
Similarly, as Vcc rises, p-channel transistors P522 and P524 in the second start-up circuit 60B turn on, supplying current to another disable node 522 and charging a connected capacitor C504, while n-channel transistor N504 remains off. When the voltage at disable node 522 reaches a predetermined level, p-channel transistor P526 turns off, n-channel transistor N506 turns on, and n-channel transistor N508 turns off, ending the pulling down of node 519. In addition, capacitor C506 charges and transistor P528 turns on, latching node 522 at the Vcc level.
During subsequent operation, node 518 is clamped at a potential equal to the sum of the base-emitter voltage (Vbe500) of bipolar transistor Q500 and the threshold voltage (Vtn500) of n-channel transistor N500. Transistor P520 remains turned off if the voltage at disable node 520 is less than the sum of this potential (Vbe500+Vtn500) and the threshold voltage (Vtp520) of transistor P520. Accordingly, the voltage at the disable node 520 is clamped at approximately Vbe500+Vtn500+Vtp520.
In this state, since transistors P516 and P518 are coupled to the first and second current mirror stages, they operate in their saturation regions, with high impedance. If the high power supply voltage Vcc varies, the variations are conducted to the source of transistor P512 through transistor P514, which remains in the on state, but the variations do not significantly affect disable node 520, because of the high impedance of transistors P516 and P518 and the cushioning effect of capacitor C502. As a result, the source-to-gate voltage of transistor P512 varies and may from time to time exceed the threshold voltage, so that transistor P512 turns on and supplies extra current to node 518. This extra current increases the gate-source bias of n-channel transistors N500 and N502, thereby increasing the current flow on paths 514 and 516, the biasing of p-channel transistors P500–P509, and the potential of node 510. If this behavior occurs repeatedly, due to periodic power-supply noise, for example, capacitor C500 gradually acquires additional charge and the bandgap reference voltage Vref drifts upward. Noise in the low power supply Vss can also cause Vref to drift.
The low-pass filters 70A, 70B in FIG. 21 are intended to solve this problem. By filtering Vcc, filter 70A reduces variations in the source potential of transistor P512 and prevents transistor P512 from turning on in synchronization with periodic noise.
The startup circuits 60A, 60B in FIG. 21 have problems other than noise, however. One problem is that, depending on the temperature characteristics of the circuit elements and the speed at which the high power supply Vcc rises when power is initially applied, the start-up operation (the pulling of nodes 518 and 519 up and down) may end too early or too late. If the start-up operation ends too early, before Vcc reaches the level necessary for constant current flow in the reference stage 50, the reference stage 50 may fail to start (fail to operate), in which case no bandgap reference voltage is generated. If the start-up operation continues too long after Vcc reaches the necessary level, the bandgap reference voltage may overshoot its intended value, and power is needlessly consumed.
Another problem is that transistors P516, P518, and P520 in start-up circuit 60A form a path through which unwanted current flows during steady-state operation.
Furthermore, the filters 70A, 70B in FIG. 21 fail to attack the root cause of the rise in the bandgap reference voltage due to power-supply noise, which is that during normal operation, disable node 520 is connected to the high power supply Vcc on a high-impedance path through transistors P516 and P518, and is held at a potential intermediate between the high power supply Vcc and the low power supply Vss, close to the switching point of p-channel transistor P520. These factors allow variations in the Vcc level to turn on transistor P512, as explained above.
Since filter 70A does not filter out low-frequency noise, it cannot completely prevent the periodic turning on of transistor P512. The reason is that transistors P516 and P518 and capacitor C502 combine with filter 70A to form an equivalent low-pass filter having a lower cut-off frequency than that of filter 70A alone. As a result, low-frequency power-supply noise that reaches the source of transistor P512 through filter 70A and transistor P514 may be cut off and fail to reach the gate of transistor P512. The consequent variations in the source-to-gate voltage of transistor P512 then turn on transistor P512, causing a gradual rise in the bandgap reference voltage Vref.
The bandgap reference voltage circuit shown in FIG. 21 thus lacks inherent immunity from power-supply noise. When power-supply noise with a frequency less than the cutoff frequency (fc) of filter 70A is present, the bandgap reference voltage may gradually increase, just as if filter 70A were absent.
The above problems of the bandgap reference voltage circuit in FIG. 21 arise from the use of the reference stage 50 to control the transistors P516, P518 and P520 that control the switching of start-up transistor P512.